1. Technical Field
The present disclosure relates generally to anti-rewet inserts for a rotating thickening device and particularly to anti-rewet inserts for roll presses commonly used in the pulp and paper industry.
2. Related Art
Pulp and paper manufacturers commonly use roll presses to wash, dewater, and thereby thicken papermaking pulp. For example, manufacturers may use a roll press to thicken pulp slurries from 2.5 percent consistency to between 30 and 50 percent consistency. Consistency is generally measured as a percent of dry fiber in a given weight of slurry.
Generally, the rolls within the roll presses have support ribs arrayed lengthwise upon a cylindrical core. The gaps between the support ribs define drainage channels and the support ribs themselves support a cylindrical deck. The deck may be a thick-walled hollow roll shell, or a series of deck rings oriented perpendicular to the support ribs. In either configuration, the hollow roll shell or the deck rings support a perforated plate. The perforated plate generally defines the outer diameter of the roll in the roll press. The gaps between the deck rings define deck channels. In configurations where the deck is a hollow roll shell, the hollow spaces within the hollow roll shell define a deck channel. In either general configuration, the deck channels provide liquid communication between the perforated plate and the drainage channels. That is, filtrate that flows through the perforated plate enters a deck channel before falling into a drainage channel.
A roll press housing typically contains one or two rolls. A roll press containing two rolls is generally known as a “twin roll press.” A trough exists between the perforated plate and the inner wall of the housing. The inner wall is usually configured such that the trough is at least partially concentric with the outer diameter of a roll; that is, the trough generally follows the contour of the roll's outer diameter at least partially. This trough may contain one or more baffles that arc gradually toward the perforated plate. The narrowest point between an end of the baffle and the perforated plate is generally known as a “nip.” A nip is generally configured to press pulp in the trough toward one or more perforated plates. In a twin roll press, the narrowest space between the perforated plates of adjacent rolls is also a “nip” and may be designated as a “twin nip” for clarity. In a twin roll press, one roll rotates in a clockwise direction and the second roll rotates in a counter-clockwise direction such that the pulp slurry is directed to the twin nip between the first and second roll.
In conventional roll presses, pulp slurry generally enters the roll press assembly though a side inlet. As the roll rotates, the pulp slurry flows through the trough and around the bottom portion of the roll. Meanwhile, the baffles, such as the baffles described in U.S. Pat. No. 8,828,189, gradually press the moving pulp slurry toward the perforated plate as the slurry moves past each nip at the end of the baffle. As the nips press the pulp, excess liquids and dissolved solids, commonly known as “filtrate”, seep through the deck perforations. As the liquids filter out of the pulp, the consistency of the pulp slurry increases to the point that baffles can form the pulp slurry into a low-consistency pulp mat around the perforated plate. Additional nips further press the pulp mat toward the perforated plate and thereby expel additional filtrate to increase further the pulp mat's solids consistency. The nips press the pulp mat or pulp suspension and force filtrate through the perforated plate and may flow across the deck channels into the drainage channels.
The filtrate then flows along the length of the drainage channel and exits the roll at either end of the drainage channel. Gravity generally assists filtrate drainage when a drainage channel rotates upwardly past the center line. The center line is an imaginary horizontal line extending from the 3 o'clock position to the 9 o'clock position on each roll.
A doctor blade is generally located above the center line. The doctor blade typically scrapes the pulp mat from the perforated plate as the roll rotates. The liberated pulp mat then exits the roll press housing for further processing. As the roll continues to rotate, the freshly exposed section of perforated plate contacts newly added pulp slurry to repeat the cycle. A side inlet usually conveys the newly added pulp slurry into the roll press housing.
The rate at which filtrate exits the drainage channels is a function of time. If a roll press rotates sufficiently slowly, most filtrate extracted from the pulp mat may exit the drainage channels when the drainage channels are above the centerline. However, at practical production rates, this is rarely accomplished.
As capacity demands increase, longer rolls, higher roll speeds, and an increase filtrate flow may be required. Additionally, demands for higher consistency pulp may increase the nip load. If designers thicken the deck, support ribs, or core to support increased nip loads, the drainage channel area may decrease, especially if designers maintain a roll diameter configured to work in existing roll press housings. Increased production may encourage a greater volume of filtrate to flow through these smaller drainage channels.
As a result, at practical production rates, not all filtrate exits the drainage channels when the drainage channels are above the center line. When this happens, the remaining filtrate can flow back into new, diluted pulp slurry when the roll rotates downwardly past the center line. In the case of a wash press, such rewetting with dirty filtrate reduces washing efficiency. The new dilute pulp slurry or pulp mat absorbs this filtrate. Returning filtrate into the pulp mat reduces the pulp mat's consistency and requires a greater nip load to expel the filtrate to achieve the desired consistency. Additionally, the increased nip load can damage the pulp mat and stress the roll, which can lead to an increased maintenance need and corresponding increase in production loss. In roll presses that are used for washing the pulp, filtrate flowing back into the pulp dirties the pulp and encourages operators to use more cleaning chemicals to achieve the desired product.
Operators have previously used rolls with anti-rewet apparatuses and inserts disposed in the drainage channels to attempt to address that rewetting that occurs as the drainage channel rotates downwardly toward the center line. These conventional anti-rewetting apparatuses however, are not configured to address the rewetting that occurs when the roll rotates upwardly toward the center line.
As a drainage channel rotates upwardly toward the center line, nips force filtrate through the perforated plate. A portion of the overall volume of the filtrate may seep through the perforated plate but fall downwardly along the deck channel without entering a drainage channel. This filtrate portion diffuses back through the perforated plate to be reabsorbed by the pulp map or pulp slurry. Rewetting in this manner likewise lowers pulp mat consistency, reduces the cleanliness of the pulp mat, and generally increases the energy required to obtain a desirable product.